In order to reduce chromium emission, tinning workers have already developed many technologies and methods for relieving chromium pollution. To summarize, the methods are divided into two types: one is for dealing with waste chromium liquid and reducing the chromium content in the waste liquid; the other is for decreasing chromium usage by recycling chromium tanning liquid or increasing the absorptive rate of chromium. These conventional ‘end-of-pipe control’ methods are passive modes with large pollution control expense, high operation costs, and high control difficulty, and thus will not bring any economic benefits to enterprises. Therefore, the chromium pollution problem shall be solved from sources, and development of clean materials used for leather tanning and free of chromium tanning agents is a hotspot currently.
A calixarene is a cyclic oligomer prepared through the reaction between phenols or derivatives thereof and aldehydes and connection via a methylene group. The calixarene features high chemical stabilities, optimal thermal stabilities, and adjustable cavity sizes and simple synthesis. Function groups such as sulfonic acid group, amino, carboxyl, and acylamino may be introduced to the calixarene through derivatization. These groups connect with groups such as amino and carboxyl on the leather collagen fiber through electrovalent bonds or hydrogen bonds, producing a tanning effect on collagen fiber, and thus improving the chemical reagent resisting effect, humidity and heat resisting stability and the like of the collagen fiber. In addition, a phenolic hydroxyl group on the lower edge of the calixarene has coordination capability, can produce hydrogen bond combination with the amino, guanidyl and peptidyl in the collagen fiber and similarly plays a tanning effect on the collagen fiber. Calixarene-synthetic raw materials are non-toxic or lowtoxic, the synthesis process is simple and easy to operate, and a synthetic product is non-toxic; therefore it is feasible to use a sulfonatocalixarene as a chromium-free tanning agent for leather tanning. The sulfonatocalixarene tanning agent belongs to a novel non-toxic and environmentally friendly chromium-free tanning agent and features a decreased chromium usage amount, which reduces chromium pollution and achieves a clean leather tanning process.